Certain organic gases known as perfluorinatedcarbons (PFCs) and hydrofluorocarbons (HFCs), emitted by industrial processes, particularly semiconductor process tools, trap infrared radiation in the atmosphere, contributing to the greenhouse effect. These gases can also decompose in the upper atmosphere where highly reactive fluorine and fluorine compounds are liberated. These interact and react with ozone, generally causing the ozone to become molecular oxygen and enter reactions forming stable compounds. There is a net removal of ozone from the atmosphere in the ozone layer, a region of the atmosphere from 18 to 48 km (12 to 30 mi) above the earth's surface. Some scientists have predicted that foreseeable destruction of the ozone layer will cause increases in ultraviolet radiation with negative health effects, damage to certain crops and to plankton and the marine food web, with an accompanying increase in carbon dioxide due to the decrease in plants and plankton. The increase in carbon dioxide gives rise to global warming because a sufficiently thick layer of carbon dioxide will also trap infrared radiation which is normally radiated away from the earth after solar heating of the earth's surface by the sun, giving rise to the "greenhouse effect".
PFCs and HFCs are greenhouse gases because of their strong infrared absorption cross sections and long atmospheric lifetimes, when not in the upper atmosphere. Thus, PFCs and HFCs behave similar to CO2 in trapping heat and causing the greenhouse effect. Because of their potential long term impact on the global climate, PFC's, HFC's, NF3 and SF6 have specifically been included in Kyoto Protocol, which aims to significantly reduce the rate of global warming gas emissions into the atmosphere.
In summary, PFCs and HFCs get trapped in the atmosphere causing an increase in the earth's temperature. Secondly, these gases absorb atmospheric UV radiation in the upper atmosphere and break down to elemental materials, including fluorine atoms, which then deplete the ozone layer. Hence, there is a danger of exposure to high levels of UV radiation on the earth's surface, causing a decrease in plants and plankton and increasing levels of atmospheric carbon dioxide, a greenhouse gas.
In U.S. Pat. No. 5,965,786 J. C. Rostaing et al. disclose use of an atmospheric pressure gas plasma for destroying PFCs and HFCs thereby eliminating greenhouse gases before the gases are released to the atmosphere. The plasma is generated by a microwave source, directing energy down a waveguide to a dielectric discharge tube where the plasma resides. Process gas is fed into the discharge tube where the PFCs and HFCs are decomposed by collisions with high energy electrons.
In U.S. Pat. No. 5,902,404 G. Fong et al. disclose use of a chamber that is open to microwave energy from a waveguide. The chamber and waveguide are operated as a resonant cavity to produce an ionized gas plasma. The purpose of the chamber is to excite a process gas for use with semiconductor manufacturing equipment where thin films are deposited on wafers.
An object of the invention was to decompose PFCs and HFCs in a discharge tube compatible with process gas handling, particularly in the semiconductor manufacturing industry.